1. Field of the Invention
The instant disclosure relates to a NAND type flash memory, and more particularly, to a NAND type flash memory for increasing data read/write reliability.
2. Description of Related Art
A flash Memory, a non-volatile memory, may keep the previously stored written data upon shutdown. In contrast to other storage media, e.g. hard disks, soft disks, magnetic tapes and so on, the flash memory has advantages of small volume, light weight, vibration-proof, low power consumption, and no mechanical movement delay in data access, therefore, is widely used as a storage media in consumer electronic devices, embedded systems, or portable computers.
There are two kinds of flash memory: an NOR flash memory and an NAND flash memory. An NOR flash memory is characteristically of low driving voltage, fast access speed, high stability, and is widely applied in portable electrical devices and communication devices such as Personal Computers, mobile phones, personal digital assistances, and set-top boxes. An NAND flash memory is specifically designed as data storage media, for example, a secure digital memory card, a compact flash card, and a memory stick card. Charges move across a floating gate relying on charge coupling which determines a threshold voltage of a transistor under the floating gate upon writing, erasing and reading. In other words, in response to an injection of electrons into the floating gate, the logical status of the floating gate turns from 1 to 0; on the contrary, in response to a move of electrons away from the floating gate, the logical status of the floating gate turns from 0 to 1.
The NAND flash memory contains a plurality of blocks, and each block has a plurality of pages wherein each page is divided into data area and spare area. The data area may contain 2048 bytes that are used for storing data. The spare area may contain 64 bytes that are used for storing error correction code. However, the flash memory may fail to change data update-in-place, in other words, erasing a block including the non-blank page is required prior to writing data into a non-blank page. If a size of written data is over an assigned block, the filled pages in the assigned block may have to be removed to other blocks, and then erasing the assigned block is performed.
There are two kinds of NAND flash memory: a multi-level cell (MLC) NAND flash memory and a single-level cell (SLC) flash memory. A cell of the MLC NAND flash memory includes a floating gate for storing various charge levels indicative of binary values 00, 01, 10, and 11. Therefore, each MLC NAND flash memory cell can store four values at one time. Conversely, the SLC NAND flash memory cell contains thinner oxide film between the floating gate and the source. Voltage is applied onto the floating gate during writing process, thereby the stored charge being driven to flow out through the source. Each SLC NAND flash memory cell may store only one-bit data, as is less than the MLC NAND flash memory cell.